Remove completely the Versioncontrol*Cache classes

As sdboyer point me out, the only missing class is VersioncontrolOperationCache, and it is because it do some statistics stuff(probably replaced in the future to use the now-only-in-our-heads activity table), __but__ there are still some places where we need to remove that!

See git grep getInstance for details:

commitlog/commitlog.module:  $vcs_accounts = VersioncontrolAccountCache::getInstance()->getAccounts(array('uids' => array($account->uid)));
commitlog/commitlog.module:  $operations = VersioncontrolOperationCache::getInstance()->getOperations($constraints, array(
commitlog/commitlog.module:  $operations = VersioncontrolOperationCache::getInstance()->getOperations($constraints, array(
hook_versioncontrol.php: *   return value of VersioncontrolAccountCache::getInstance()->getAccounts(). Entries in this list
hook_versioncontrol.php: *   a structured array like the one returned by VersioncontrolAccountCache::getInstance()->getAccounts().
includes/VersioncontrolOperationCache.php:   * VersioncontrolOperationCache::getInstance()->getOperations().
includes/VersioncontrolOperationCache.php:  public static function getInstance() {
includes/VersioncontrolOperationCache.php:    $repositories = VersioncontrolRepositoryCache::getInstance()->getRepositories(array('repo_ids' => $repo_ids));
includes/VersioncontrolOperationCache.php:   * Convenience function, calling VersioncontrolCache::getInstance()->_get_operations() with a preset
includes/VersioncontrolRepository.php:    // $operations = VersioncontrolOperationCache::getInstance()->getOperations(array('repo_ids' => array($this->repo_id)));
includes/interfaces.inc:   *   VersioncontrolAccountCache::getInstance()->getAccounts().  All
includes/versioncontrol.rules.inc:    $operations = VersioncontrolOperationCache::getInstance()->getOperations(array('vc_op_ids' => array($vc_op_id)));
versioncontrol.admin.inc:  $statistics = VersioncontrolOperationCache::getInstance()->getStatistics(array(
versioncontrol.module: *   Will be passed on to VersioncontrolAccountCache::getInstance()->getAccounts(), see the
versioncontrol_account_status/versioncontrol_account_status.module: *   a structured array like the one returned by VersioncontrolAccountCache::getInstance()->getAccounts().
versioncontrol_account_status/versioncontrol_account_status.module: *   return value of VersioncontrolAccountCache::getInstance()->getAccounts(). Entries in this list
versioncontrol_fakevcs/includes/classes.inc:      $commit_operations = VersioncontrolOperationCache::getInstance()->getCommits($constraints);
versioncontrol_views/handlers/versioncontrol_plugin_row_operation.inc:    $this->operations = VersioncontrolOperationCache::getInstance()->getOperations($constraints);

Marking as bug, as some code is actually called.